High Risk CTV Asymmetry Predicts Dosimetric Benefit with Use of Combined Intracavitary/Interstitial Brachytherapy Applicators for Cervical Cancer

Purpose/Objective(s)Compared to intracavitary (IC) brachytherapy applicators, combined intracavitary/interstitial (IC/IS) applicators provide greater dose to the High-risk CTV (CTVHR) with similar doses to organ at risk (OAR) structures. Despite increasing use, there is little data to guide clinicians as to which patients are most likely to benefit from use of IC/IS applicators. Recent data from retroEMBRACE suggests CTVHR volume greater than 30 cm3(cc) is associated with increased likelihood of benefit. In previous work, we provided initial evidence that asymmetry of the CTVHR may correlate with benefit to use of IC/IS applicators. Here, we provide an expanded analysis, hypothesizing that the degree of asymmetry of the CTVHR with respect to location of the tandem rod may better correlate with benefit than CTVHR volume. To test this hypothesis, we performed a dosimetric study of image guided adaptive brachytherapy treatment plans modeled with IC versus IC/IS applicators.Materials/MethodsTreatment planning MRI or CT datasets for thirty patients treated with IC or IC/IS brachytherapy applicators as part of definitive therapy for FIGO 2018 stage IB3 to IIIC cervical cancer were acquired. IC and IC/IS HDR brachytherapy treatment plans were generated for each dataset using a radiotherapy treatment planning system software from a precision radiation medicine company. For patients treated with IC applicators, needles were virtually digitized and dwell positions modeled based on ovoid location to create IC/IS plans. For patients treated with IC/IS applicators, needle dwell positions were removed and plans re-optimized based on tandem and ovoid dwell positions only. All plans were optimized to minimize dose to the D2cc volume of OARs while reaching an equivalent dose to the D90 of the CTVHR. OAR (bladder, rectum, and sigmoid colon), and CTVHR volumes were not modified from the original administered treatments. The maximum distance (dmax) between the tandem rod and distal edge of the CTVHR in the axial plan at the level of greatest CTVHR asymmetry, as determined visually by the author (DS), was obtained for each dataset. Conformity index (CI) was defined as the ratio of the prescription isodose volume to the CTVHR, and was computed for each plan. Spearman correlation coefficients were used to compare the CI improvement (%CI) in IC/IS versus IC plans, OAR dose reductions, and to measure the correlation between the %CI improvement and the asymmetry surrogate dmax.ResultsMean %CI improvement in IC/IS plans compared to IC plans was 20.6%. The %CI improvement in IC/IS plans demonstrated a significant correlation with D2cc dose reduction for bladder (p<0.01), rectum (p<0.01), and sigmoid (p<0.01). The %CI improvement in IC/IS plans demonstrated a significant correlation with dmax (p<0.01). We found no correlation between %CI improvement and volume of the CTVHR.ConclusionDuring cervical cancer brachytherapy, greater CTVHR asymmetry, as measured by dmax, correlates with likelihood of benefit from use of IC/IS versus IC brachytherapy applicators.